The present invention is in the field of electrochemical devices and more particularly relates to flexible ceramic sheets for solid electrolytes and electrolyte/electrode assemblies for devices such as fuel cells.
U.S. Pat. No. 5,089,455 describes strong, thin, flexible ceramic sheets and tapes of various compositions, and methods for making them. As taught in U.S. Pat. No. 5,273,837, such sheets can be used to provide solid oxide electrolytes and other components for fuel cells that exhibit improved resistance to thermal shock damage due in part to the flexibility and high strength of the ceramic sheets. Further, U.S. Pat. No. 5,519,191 describes the incorporation of thin ceramic sheets into fluid heating structures of corrugated shape that include thin conductive metal layers as electrical heating elements.
Curved electrode and electrolyte designs that reduce the thermal stresses arising during the normal operation of fuel cells are disclosed in published PCT patent application WO99/44254. The use of corrugated planar electrode/electrolyte sheets to control such stresses is proposed by K. Tomida et al. in xe2x80x9cPreparation of Solid Electrolyte Thin Films for Relaxing Thermal Stressesxe2x80x9d, Proceedings of the Third International Symposium on Solid Oxide Fuel Cells, Proceedings Volume 93-4, pages 74-81, Singhal and Iwahara, Editors, The Electrochemical Society, Inc. (1993).
Substantially planar electrolyte sheets supporting cathodic and anodic electrode layers have been proposed for use in a number of different fuel cell configurations, including configurations that may be characterized as stacked fuel cell designs. In one such stacked design, each planar electrode/electrolyte sub-unit is bonded to and edge-supported by a framing manifold structure, with multiple frames and sub-units being stacked and electrically interconnected in parallel or series to provide the fuel cell output current or voltage required for the particular application of interest.
In this and similar manifolded fuel cell arrangements, even perfect thermal expansion matching of the electrolyte/electrode sheets to the supporting manifold structure does not avoid thermal cycling stress. This is because the manifold structures typically have much higher thermal mass than the sheets, and heat and cool sufficiently more slowly than the electrolyte/electrode sheets that the electrolyte/electrode sheets can be put into severe tension in many sheet directions at once regardless of the extent to which thermal expansion matching is employed.
Unfortunately, the known materials and designs for thin ceramic fuel cell electrolytes do not provide the level of thermal durability necessary to insure dependable fuel cell operation in stacked and other configurations during the extended temperature cycling that cannot be avoided in normal service. In particular, prior art electrolytes do not provide the requisite combination of high multiaxial strain tolerance and high resistance to damage under large strains that will be needed to secure dependable long-term service in fuel cells.
The present invention provides highly strain tolerant ceramic electrolyte layers wherein the electrolyte is formed of a strong, thin ceramic sheet incorporating a two-dimensional surface indentation pattern. For example, flexible ceramic sheet having a surface indentation pattern providing a strain tolerance of not less than 0.5% in any direction in the sheet plane, more preferably a strain tolerance of at least 1% in any direction in the sheet plane, can readily be provided by means hereinafter described.
Useful indentation patterns are those that impart a very high multi-axial strain tolerance to the sheet, within the plane of the sheet, without introducing stress concentrators that reduce sheet strength. Examples of suitable indentation patterns are those comprising multidirectional corrugations or waves, protrusions or indentations of circular, polygonal, or other cross-section, and other contiguous or overlapping indentations or protrusions that do not introduce sharp sheet curvature and do not alter the generally planar configuration of the sheet. One-dimensional patterns, such as single-direction corrugations that provide only uni-axial strain tolerance, are not useful.
The preferred indentation patterns allow not only large in-plane effective strains but also large elastic deformations normal to the plane of the sheet. This permits the sheets to withstand large thermal gradients and large thermal expansion differentials from associated other fuel cell components without risking electrolyte fracture and loss of effective current generation.
The invention further includes a process for making thin, strain-tolerant ceramic sheet for electrolyte and electrolyte/electrode fabrication. In general the process involves the steps of forming a thin cohesive green sheet layer on a suitable fugitive support, forming patterned indentations in the sheet while in the green state, and then consolidating the sheet with its impressed indentation pattern by sintering to remove binders and any fugitive supports. Methods that can be used to impress the desired indentation pattern in the green sheet include vacuum forming, pressing, roll pressing, embossing, or other conventional surface shaping procedures.
Strain tolerant electrolyte sheets produced as described can be employed in a variety of different fuel cell configurations, but are of particular value in planar stacked fuel cell designs. This is because the strain-tolerant electrolyte sheets of the invention offer much higher resistance to mechanical failure under temperature cycling conditions, particularly in an edge-supported electrolyte configuration, than do conventional corrugated or other electrolyte sheet designs.